At least 34 civilians, including nine children, have been killed in a Russian air raid on Euphrates River ferries near Syria's Deir Az Zor city, according to a monitoring group.

Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights, said the air strikes on Sunday targeted "more than 40 ferries" that left al-Boulil town southwest of Deir Az Zor for the eastern shore of the Euphrates.

No immediate response from Russian authorities was available.

Sunday's attack came as Syrian troops pressed an offensive against Islamic State of Iraq and the Levant (ISIL) across Deir Az Zor province with Russian air cover.

According to the War Media channel operated by Syrian government ally Hezbollah, Syrian troops seized full control of the 450km road linking the capital, Damascus, to Deir Az Zor for the first time in four years.

"The Syrian army and its allies now control the entire international highway between Deir Az Zor and Damascus, through the cities of al-Sukhna and Palmyra," it said, referring to other central Syrian cities recaptured from ISIL (also known as ISIS).